Diffractive optical element and method for manufacturing same

ABSTRACT

A diffractive optical element includes at least a first diffractive element and a second diffractive element. The conditional expression 0.5≦D/DS≦0.9 is preferably satisfied where DS denotes the summation of the optimum designed groove height of the first diffractive element d1S and that of the second diffractive element d2S, and D denotes the summation of an actual groove height of the first diffractive element d1 and that of the second diffractive element d2. At least one of the first diffractive element and the second diffractive element is made of glass. At least one of the first diffractive element and the second diffractive element is made of resin. The optimum designed value of groove heights of the diffractive optical element are determined so as to satisfy a condition for correcting chromatic aberration at both d-line and g-line.

INCORPORATION BY REFERENCE

[0001] The disclosures of the following priority applications are hereinincorporated by reference:

[0002] Japanese Patent Application No. 2002-026309 filed Feb. 4, 2002,

[0003] Japanese Patent Application No. 2002-026310 filed Feb. 4, 2002,and

[0004] Japanese Patent Application No.2002-065744 filed Mar. 11, 2002.

BACKGROUND OF THE INVENTION

[0005] 1. Field of the Invention

[0006] The present invention relates to a diffractive optical elementfor producing diffracted light flux relative to an incident light, andmore particularly to a diffractive optical element constructed bylaminating a plurality of diffractive elements.

[0007] 2. Description of Related Art

[0008] A diffractive optical element is an optical element havinglattice structure of slits or grooves spacing at even intervals withseveral hundreds lines per a small distance (about 1 mm), and it hascharacteristic that when a light is incident to it, it producesdiffracted light flux in a direction determined by the wavelength of thelight and the separation (pitch) of slits or grooves. Diffractiveoptical elements like this are used in various kinds of optical systems,for example, an optical element used as a lens for converging a specificorder of diffracted light into a point is known.

[0009] In diffractive optical elements like this, a diffractive opticalelement called a plurality of layers type has been proposed. Thediffractive optical element of this type has structure laminating aplurality of diffractive elements having a saw-tooth shape surface in aform appressed or separated with each other. It has a characteristichaving high diffractive efficiency over almost entire range of arequired wide wavelength range (for example, whole visible light range),in other words, good spectral characteristic.

[0010] As shown in FIG. 9, structure of a laminated type diffractiveoptical element is generally composed of a first diffractive element 310made of a first material, and a second diffractive element 320 made of asecond material having different refractive index and dispersion fromthose of the first material. The faces of respective diffractiveelements facing each other are saw-tooth shape surfaces as shown in thedrawing. Here, in order to satisfy the condition for correctingchromatic aberration at predetermined two wavelengths, the groove heightd1 of the first diffractive element 310 is set to a predetermined valueand that d2 of the second diffractive element 320 is set to anotherpredetermined value. Accordingly, diffraction efficiencies regarding thepredetermined two wavelengths become 1.0 and considerably highdiffractive efficiency can be obtained at the other wavelength. In thetransparent type diffractive optical element, diffractive efficiency isdefined as a ratio η_(A)(=A₁/A₀) of amplitude of a first orderdiffracted light A₁ to that of an incident light A₀. Alternatively,diffractive efficiency is defined as a ratio η_(I)(=I₁/I₀) of intensityof a first order diffracted light I₁ to that of an incident light I₀. Bythe way, the square of light amplitude generally indicates lightintensity (for example, A₁ ²=I₁). In this case, light loss caused byabsorption or scattering is supposed not to exist.

[0011] However, in a conventional diffractive optical element, in orderto satisfy the condition for correcting chromatic aberration atpredetermined two wavelengths, the groove height d1 of the firstdiffractive element 310 and that d2 of the second diffractive element320 become considerably larger relative to the case each diffractiveelement is used independently. The groove height of a universallywell-known diffractive optical element (a single layer diffractiveoptical element) is about 1 μm. On the other hand, in almost all thesemulti-layer type diffractive optical elements, the optimum designedvalue of the groove height of a diffractive optical element, in otherwords, the total groove height D (=d1+d2) of all elements becomes morethan 10 μm.

[0012] In the case of the groove height of whole diffractive opticalelement becomes large, even if an incident light slightly tilts from thereference optical axis, light flux properly passing through bothdiffractive optical elements 310 and 320 decreases. Accordingly, it hasbeen a problem that when a rate of decrease in diffractive efficiencyrelative to variation in the incident angle of the incident light(hereinafter called angular characteristic) is applied, the angularcharacteristic of the multi-layer type diffractive optical elementdecreases largely relative to an ordinary diffractive optical element.Moreover, it has become difficult to form grooves of the diffractiveelement in accordance with the width of the pitch thereof.

[0013] Furthermore, in a conventional multi-layer type diffractiveoptical element shown in FIG. 9, since the groove height d1 of the firstdiffractive element 310 differs from that d2 of the second diffractiveelement 320, the diffractive elements 310 and 320 have to be madeseparately with the same procedure. Finally, both diffractive elements310 and 320 must be precisely positioned, so that it becomes difficultto manufacture.

SUMMARY OF THE INVENTION

[0014] The present invention is made in view of the aforementionedproblems and has an object to provide a diffractive optical elementcapable of enhancing angular characteristic without severelydeteriorating spectral characteristic, improving productivity of thediffractive optical element.

[0015] It is also an object of the present invention to provide a methodfor manufacturing a multi-layer type diffractive optical element to beeasily manufactured.

[0016] According to an aspect of the present invention, a diffractiveoptical element includes at least a first diffractive element and asecond diffractive element. In this diffractive optical element, thefollowing conditional expression is satisfied:

0.5≦D/DS≦0.9

[0017] where DS denotes a summation of the optimum designed grooveheights of the diffractive optical elements and D denotes a summation ofactual groove heights of the diffractive optical elements.

[0018] In one preferred embodiment of the present invention, at leastone of the first diffractive element and the second diffractive elementis made of glass.

[0019] In other preferred embodiment of the present invention, at leastone of the first diffractive element and the second diffractive elementis made of resin.

[0020] In one preferred embodiment of the present invention, the optimumdesigned groove heights of the diffractive optical element aredetermined so as to satisfy a condition for correcting chromaticaberration at both d-line and g-line.

[0021] According to another aspect of the present invention, adiffractive optical element includes a first diffractive element and asecond diffractive element. The first and second diffractive elementsare made of different materials with each other, closely laminated witheach other to form grooves of a diffraction grating having apredetermined shape at the cemented surface of both the first and thesecond diffractive elements. One of these diffractive elements is madeof material for molding and the other diffractive element is made ofultraviolet-curable resin. Further, the following conditional expressionis satisfied:

7.0 μm≦h≦18.0 μm

[0022] where h denotes the groove height of the diffraction grating.

[0023] According to another aspect of the present invention, adiffractive optical element includes a first diffractive element and asecond diffractive element. The first and second diffractive elementsare made of different materials with each other, closely laminated witheach other to form grooves of a diffraction grating having apredetermined shape at the cemented surface of both the first and thesecond diffractive elements. One of these diffractive elements is madeof material for molding and the other diffractive element is made ofultraviolet-curable resin. A pitch of the grooves of the diffractiongrating is 70 μm or more.

[0024] In one preferred embodiment of the present invention, thematerial for molding is glass and the following conditional expressionsare preferably satisfied:

1.55≦ndG≦1.65

55≦νdG≦65

1.50≦ndR≦1.60

νdR≦40

[0025] where ndG denotes refractive index of the glass at d-line, νdGdenotes Abbe number of the glass, ndR denotes refractive index of theultraviolet-curable resin at d-line, and νdR denotes Abbe number of theresin.

[0026] In one preferred embodiment of the present invention, thematerial for molding is glass and the following conditional expressionsare preferably satisfied:

1.63≦ndG≦1.73

50≦νdG≦60

1.58≦ndR≦1.68

νdR≦35

[0027] where ndG denotes refractive index of the glass at d-line, νdGdenotes Abbe number of the glass, ndR denotes refractive index of theultraviolet-curable resin at d-line, and νdR denotes Abbe number of theresin.

[0028] According to another aspect of the present invention, adiffractive optical element includes a first transparent member on whichgrooves of a diffraction grating is formed, a second transparent memberhaving a plane or curved surface, and an adhesive agent for cementingthe grooves of the diffraction grating of the first transparent memberwith the plane or curved surface of the second transparent member.

[0029] In one preferred embodiment of the present invention, the firsttransparent member is made of material for molding. The adhesive agentis made of ultraviolet-curable resin.

[0030] In one preferred embodiment of the present invention, thefollowing conditional expressions are preferably satisfied:

1.55≦ndG≦1.70

50≦νdG≦65

1.50≦ndR≦1.65

νdR≦45

[0031] where ndG denotes refractive index at d-line of the firsttransparent member, νdG denotes Abbe number of the first transparentmember, ndR denotes refractive index at d-line of the adhesive agent andνdR denotes Abbe number of the adhesive agent.

[0032] In one preferred embodiment of the present invention, thefollowing conditional expression is preferably satisfied:

h≦16.0 μm

[0033] where h denotes the groove height of the diffraction grating.

[0034] In one preferred embodiment of the present invention, a pitch ofthe grooves of the diffraction grating is preferably 80 μm or more.

[0035] According to another aspect of the present invention, a methodfor manufacturing a diffractive optical element includes steps of afirst step that forms grooves of a diffraction grating on a surface of afirst transparent member, a second step that drips an adhesive agentonto the grooves of the diffraction grating on the surface of the firsttransparent member, a third step that attaches a second transparentmember having a plane or curved surface to the adhesive agent, and afourth step that cures the adhesive agent.

[0036] In one preferred embodiment of the present invention, the firststep is composed of a hardening step that the first transparent memberis pressed, formed and hardened by using a mold on which the grooves ofthe diffraction grating are formed and a removing step that removes thehardened first transparent member from the mold.

[0037] In one preferred embodiment of the present invention, the fourthstep is composed of an irradiating step that irradiates the adhesiveagent with an ultraviolet light.

[0038] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

[0040]FIG. 1A is a sectional view showing a multi-layer type diffractiveoptical element composed of separated two layers according to a firstembodiment of the present invention and

[0041]FIG. 1B is a sectional view showing a standard diffractive opticalelement in accordance with the multi-layer type diffractive opticalelement.

[0042]FIG. 2 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated three layers according toa first embodiment of the present invention.

[0043]FIG. 3 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated two layers according to afirst embodiment of the present invention.

[0044]FIG. 4 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated two layers according to asecond embodiment of the present invention.

[0045] FIGS. 5A-5G are sectional views showing manufacturing processesof the diffractive optical element according to the second embodiment ofthe present invention.

[0046]FIG. 6 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated three layers according toa third embodiment of the present invention.

[0047] FIGS. 7A-7G are sectional views showing manufacturing processesof the diffractive optical element according to the third embodiment ofthe present invention.

[0048]FIGS. 8A and 8B are sectional views of diffractive opticalelements according to modified examples of the third embodiment of thepresent invention, wherein FIG. 8A is a case that a surface of a secondtransparent member come in contact with adhesive has a convex shapefacing to the diffraction grating side and wherein FIG. 8B is a casethat a surface of a second transparent member come in contact withadhesive has a concave shape facing to the diffraction grating side.

[0049]FIG. 9 is a sectional view showing a conventional diffractiveoptical element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] [First Embodiment]

[0051] At first, a first embodiment of the present invention isexplained with reference to the accompanying drawings. FIG. 1A is asectional view showing a multi-layer type diffractive optical elementcomposed of separated two layers according to a first embodiment of thepresent invention. In the diffractive optical element 1 according to thefirst embodiment of the present invention as shown in FIG. 1A, a firstdiffractive element 10 arranged to the light-incident side and a seconddiffractive element 20 arranged to the light-exit side are facing witheach other separating with a predetermined distance. The surfaces ofboth diffractive elements 10 and 20 facing with each other are formedinto a sawtooth shape. Moreover, these first and second diffractiveelements 10 and 20 are made of different materials with each other andhave different refractive indices and dispersion. FIG. 1B is a sectionalview showing the diffractive optical element 1S obtained when thediffractive optical element is designed with the optimum conditions byusing a first and a second diffractive elements 10S and 20S similar tothose 10 and 20 used in the diffractive optical element 1 according tothe first embodiment of the present invention. Hereinafter thediffractive optical element 1S is called “the standard diffractiveoptical element” in opposition to the diffractive optical element 1according to the first embodiment of the present invention.

[0052] As shown in FIGS. 1A and 1B, groove heights of the multi-layertype diffractive optical elements 1 and 1S composed of separated twolayers are obtained as described below. In the standard diffractiveoptical element 1S, the groove height DS is obtained as a summation ofthe groove height d1S of the first diffractive element 10S and that d2Sof the second diffractive element 20S, namely DS=d1S+d2S. In thediffractive optical element 1, the groove height D is obtained as asummation of the groove height d1 of the first diffractive element 10and that d2 of the second diffractive element 20, namely D=d1+d2.

[0053] Here, the groove height d1S of the first diffractive element 10Sand that d2S of the second diffractive element 20S is derived by solvingthe following simultaneous equations (1) and (2):

d1S·(n11−1)−d2S·(n21−1)=λ1 (1)

d1S·(n12−1)−d2S·(n22−1)=λ2 (2)

[0054] where λ1 and λ2 denote two wavelengths at which chromaticaberration correction is carried out, n11 denotes the refractive indexof the first diffractive element d1S at wavelength λ1, n12 denotes therefractive index of the second diffractive element d2S at wavelength λ1,n21 denotes the refractive index of the second diffractive element d1Sat wavelength λ1, and n22 denotes the refractive index of the seconddiffractive element d1S at wavelength λ2.

[0055] In the first embodiment of the present invention, relative to thegroove height DS (a summation of optimum designed value of the grooveheight) of the standard diffractive optical element 1S obtained by thesummation of d1S and d2S, the groove height D (=d1+d2) of thediffractive optical element 1 of the first embodiment is determined bysetting the groove height d1 of the first diffractive element and thatd2 of the second diffractive element so as to satisfy the followingconditional expression (3):

0.5≦D/DS≦0.9   (3)

[0056] An example of the specific procedure to determine the grooveheight D of the diffractive optical element 1 according to the firstembodiment of the present invention is explained. After obtaining thegroove height d1S of the diffractive element 10S and that d2S of thediffractive element 20S of the standard diffractive optical element 1Sby solving simultaneous equations (1) and (2), at first, the grooveheight d1 of the first diffractive element 10 of the diffractive opticalelement 1 according to the first embodiment of the present invention isdetermined by multiplying the groove height d1S of the first diffractiveelement 10S by the value D/DS (the value can be selected arbitrarily)satisfying the equation (3). Then, the groove height d2 of the seconddiffractive element 20 of the diffractive optical element 1 according tothe first embodiment of the present invention is temporarily set to avalue which gives high diffraction efficiency (e.g. 0.98 or more) atusing wavelength range (e.g. visible light range).

[0057] The groove height d1 of the first diffractive element 10 and thatd2 of the second diffractive element 20 of the diffractive opticalelement 1 according to the first embodiment of the present invention areset in this manner, so the groove height D (=d1+d2) of the diffractiveoptical element 1 according to the first embodiment of the presentinvention is derived from both values d1 and d2, and whether the value Dsatisfies the conditional expression (3) or not is confirmed. When thegroove height D does not satisfy conditional expression (3), theprocedure for temporarily setting the groove height d2 of the seconddiffractive element 20 is tried again. When the groove height Dsatisfies conditional expression (3), the temporarily set value of d2 isset (formally set) as the groove height of the second diffractiveelement 20.

[0058] The diffractive optical element 1 composed of the firstdiffractive element 10 having the groove height d1 and the seconddiffractive element 20 having the second groove height d2 obtained(formally set) in accordance with the above-described procedure does notseverely deteriorate diffractive efficiency relative to the usingwavelength range and improves angular characteristics (see detaileddescription of preferred embodiments).

[0059] In the conditional expression (3), the reason why the lower limitof the groove height of the diffractive optical element 1 according tothe first embodiment of the present invention is 0.5 times as high asthe groove height DS (optimum designed value of the groove height) ofthe standard diffractive optical element 1S is that when the ratio D/DSfalls below the lower limit of conditional expression (3), degradationof diffractive efficiency in the short wavelength range (around g-line)and in the long wavelength range (around C-line) becomes large ( inother words, spectral characteristics becomes worse) and amount of flarelight increases. It is difficult to apply such a diffractive opticalelement to an imaging lens of an imaging device.

[0060] In conditional expression (3), the reason why the upper limit ofthe groove height of the diffractive optical element 1 according to thefirst embodiment of the present invention is 0.9 times as high as thegroove height DS of the standard diffractive optical element 1S is thatwhen the ratio D/DS exceeds the upper limit of conditional expression(3), the groove height becomes almost same as that (namely, the optimumdesigned value) of the standard diffractive optical element 1S, so thatthe purpose of the present invention disappears.

[0061] When the lower limit of conditional expression (3) is set to 0.55larger than 0.5, better spectral characteristics can be obtained andwhen it is set to 0.6 even better spectral characteristics can beobtained. Moreover, when the upper limit of conditional expression (3)is set to 0.85 other than 0.9, better angular characteristics can beobtained and when it is set to 0.8, even better angular characteristicscan be obtained.

[0062] It is preferable that at least one of the first diffractiveelement 10 and the second diffractive element 20 of the diffractiveoptical element 1 according to the first embodiment of the presentinvention is made of glass. Since glass has a lot of kinds, you canchoose from wide option. Moreover, it is preferable that at least one ofthe aforementioned both diffractive optical elements 10 and 20 is madeof resin. Although resin having fewer kinds has narrow choices, resincan be easily processed and has good productivity.

[0063] The optimum designed groove height DS of the diffractive opticalelement 1 according to the first embodiment of the present invention ispreferably designed to satisfy the condition of correcting chromaticaberration at d-line and g-line. In this way, the diffractive opticalelement 1 according to the first embodiment of the present invention canbe applied to almost all visible wavelengths, so that it becomes easy tobe used for an imaging lens of a photographic camera using a whitelight. The first embodiment uses diffraction efficiency (η_(A)).

EXAMPLE 1

[0064] Here, a glass material of BK7 (made by Schott Glas) is used asthe first diffractive element 10 and a glass material F2 (made by SchottGlas) is used as the second diffractive element 20. Correction ofchromatic aberration is carried out at d-line (587.6 nm) and g-line(435.8 nm). Various values of refractive index are listed in Table 1.TABLE 1 λ1 = 587.6 nm λ2 = 435.8 nm BK7 n11 = 1.5168 n12 = 1.52668 F2n21 = 1.62004 n22 = 1.64202

[0065] The (optimum designed values) of the groove heights d1S and d2Sare calculated by substituting these values for the equations (1) and(2) as shown Table 2. TABLE 2 d1S (optimum) = 20453 nm d2S (optimum) =16100 nm DS = 36553 nm

Example 1-1

[0066] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 18400 nm which is 0.90times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 14400 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.897, so theequation (3) is satisfied.

Example 1-2

[0067] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 15400 nm which is 0.75times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 11900 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.747, so theequation (3) is satisfied.

Example 1-3

[0068] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 10500 nm which is 0.51times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 7850 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.502, so the equation(3) is satisfied.

[0069] The values of diffraction efficiency of the diffractive opticalelements according to Example 1 are listed in Table 3 where the pitch ofthe grooves is 0.1 mm, incident light is d-line, and incident angle is+5°. Moreover, in order to confirm that diffraction efficiency does notsharply decrease at other wavelength, diffraction efficiency at C-lineis also shown. The value of diffraction efficiency at C-line is atnormal incidence. TABLE 3 diffraction efficiency d-line C-line (+5°)(normal) optimum designed value 0.57 0.994 (Example 1-1) 0.68 0.991(Example 1-2) 0.80 0.989 (Example 1-3) 0.92 0.973

EXAMPLE 2

[0070] Here, a glass material of FK52 (made by Schott Glas) is used asthe first diffractive element 10 and a glass material BaF4 (made bySchott Glas) is used as the second diffractive element 20. Correction ofchromatic aberration is carried out at d-line (587.6 nm) and g-line(435.8 nm). Various values of refractive index are listed in Table 4.TABLE 4 λ1 = 587.6 nm λ2 = 435.8 nm FK52 n11 = 1.48605 n12 = 1.49338BaF4 n21 = 1.60562 n22 = 1.62318

[0071] The (optimum designed values) of the groove height d1S and d2Sare calculated by substituting these values for the equations (1) and(2) as shown Table 5. TABLE 5 d1S (optimum) = 24965 nm d2S (optimum) =19066 nm DS = 44031 nm

Example 2-1

[0072] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 20000 nm which is 0.80times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 15100 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.797, so theequation (3) is satisfied.

Example 2-2

[0073] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 17500 nm which is 0.70times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 13100 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.695, so theequation (3) is satisfied.

Example 2-3

[0074] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 15000 nm which is 0.60times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 11100 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.593, so theequation (3) is satisfied.

[0075] The values of diffraction efficiency of the diffractive opticalelements according to Example 2 are listed in Table 6 where the pitch ofthe grooves is 0.1 mm, incident light is d-line, and incident angle is+5°. Moreover, in order to confirm that diffraction efficiency does notsharply decrease at other wavelength, diffraction efficiency at C-lineis also shown. The value of diffraction efficiency at C-line is atnormal incidence. TABLE 6 diffraction efficiency d-line C-line (+5°)(normal) optimum designed value 0.28 0.995 (Example 2-1) 0.62 0.989(Example 2-2) 0.74 0.985 (Example 2-3) 0.81 0.982

EXAMPLE 3

[0076] Here, a glass material of SK11 (made by Schott Glas) is used asthe first diffractive element 10 and a glass material SF4 (made bySchott Glas) is used as the second diffractive element 20. Correction ofchromatic aberration is carried out at d-line (587.6 nm) and g-line(435.8 nm). Various values of refractive index are listed in Table 7.TABLE 7 λ1 = 587.6 nm λ2 = 435.8 nm SK11 n11 = 1.56384 n12 = 1.57530 SF4n21 = 1.75520 n22 = 1.79121

[0077] The (optimum designed values) of the groove height d1S and d2Sare calculated by substituting these values for the equations (1) and(2) as shown Table 8. TABLE 8 d1S (optimum) = 11657 nm d2S (optimum) = 7925 nm DS = 19582 nm

Example 3-1

[0078] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 10000 nm which is 0.85times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 6700 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.853, so the equation(3) is satisfied.

Example 3-2

[0079] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 9000 nm which is 0.75times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 5950 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.763, so the equation(3) is satisfied.

Example 3-3

[0080] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 7500 nm which is 0.65times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 4850 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.631, so the equation(3) is satisfied.

[0081] The values of diffraction efficiency of the diffractive opticalelements according to Example 3 are listed in Table 9 where the pitch ofthe grooves is 0.1 mm, incident light is d-line, and incident angle is+5°. Moreover, in order to confirm that diffraction efficiency does notsharply decrease at other wavelength, diffraction efficiency at C-lineis also shown. The value of diffraction efficiency at C-line is atnormal incidence. TABLE 9 diffraction efficiency d-line C-line (+5°)(normal) optimum designed value 0.91 0.994 (Example 3-1) 0.94 0.989(Example 3-2) 0.96 0.989 (Example 3-3) 0.97 0.980

EXAMPLE 4

[0082] Here, a material of PMMA is used as the first diffractive element10 and a resin material A is used as the second diffractive element 20.Correction of chromatic aberration is carried out at d-line (587.6 nm)and g-line (435.8 nm). Various values of refractive index are listed inTable 10. TABLE 10 λ1 = 587.6 nm λ2 = 435.8 nm PMMA n11 = 1.4908 n12 =1.5016 Resin A n21 = 1.7046 n22 = 1.7336

[0083] The (optimum designed values) of the groove height d1S and d2Sare calculated by substituting these values for the equations (1) and(2) as shown Table 11. TABLE 11 d1S (optimum) = 18520 nm d2S (optimum) =12066 nm DS = 30586 nm

Example 4-1

[0084] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 16000 nm which is 0.86times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 10310 nm which gives high diffraction efficiencyover the visible wavelength range. The ratio D/DS is 0.860, so theequation (3) is satisfied.

Example 4-2

[0085] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 13000 nm which is 0.70times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 8240 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.694, so the equation(3) is satisfied.

Example 4-3

[0086] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 9700 nm which is 0.52times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 5970 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.512, so the equation(3) is satisfied.

[0087] The values of diffraction efficiency of the diffractive opticalelements according to Example 4 are listed in Table 12 where the pitchof the grooves is 0.1 mm, incident light is d-line, and incident angleis +50. Moreover, in order to confirm that diffraction efficiency doesnot sharply decrease at other wavelength, diffraction efficiency atC-line is also shown. The value of diffraction efficiency at C-line isat normal incidence. TABLE 12 diffraction efficiency d-line C-line (+5°)(normal) optimum designed value 0.74 0.995 (Example 4-1) 0.82 0.993(Example 4-2) 0.89 0.986 (Example 4-3) 0.95 0.971

EXAMPLE 5

[0088] Here, a resin material of an ultraviolet-curable resin B is usedas the first diffractive element 10 and a resin material of anultraviolet-curable resin C is used as the second diffractive element20. Correction of chromatic aberration is carried out at d-line (587.6nm) and g-line (435.8 nm). Various values of refractive index are listedin Table 13. TABLE 13 λ1 = 587.6 nm λ2 = 435.8 nm Resin B n11 = 1.524n12 = 1.537 Resin C n21 = 1.635 n22 = 1.674

[0089] The (optimum designed values) of the groove height d1S and d2Sare calculated by substituting these values for the equations (1) and(2) as shown Table 14. TABLE 14 d1S (optimum) =  9500 nm d2S (optimum) = 6900 nm DS = 16400 nm

Example 5-1

[0090] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 7600 nm which is 0.80times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 5360 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.790, so the equation(3) is satisfied.

Example 5-2

[0091] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 6700 nm which is 0.70times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 4630 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.691, so the equation(3) is satisfied.

Example 5-3

[0092] In this example, the groove height d1 of the first diffractiveelement 10 of the diffractive optical element 1 according to the firstembodiment of the present invention is set to 5900 nm which is 0.62times as high as the optimum designed groove height d1S. The grooveheight d2 is set to 3980 nm which gives high diffraction efficiency overthe visible wavelength range. The ratio D/DS is 0.602, so the equation(3) is satisfied.

[0093] The values of diffraction efficiency of the diffractive opticalelements according to Example 5 are listed in Table 15 where the pitchof the grooves is 0.1 mm, incident light is d-line, and incident angleis +5°. Moreover, in order to confirm that diffraction efficiency doesnot sharply decrease at other wavelength, diffraction efficiency atC-line is also shown. The value of diffraction efficiency at C-line isat normal incidence. TABLE 15 diffraction efficiency d-line C-line (+5°)(normal) optimum designed value 0.94 0.996 (Example 5-1) 0.96 0.988(Example 5-2) 0.97 0.982 (Example 5-3) 0.98 0.977

[0094] As is shown from the above results, when the summation D(=d1+d2)of the groove heights d1 of the first diffractive element 10 and that d2of the second diffractive element 20 of the diffractive optical element1 according to the first embodiment of the present invention satisfiesthe equation (3), the diffractive optical element 1 gives higherdiffraction efficiency at incident angle +5° than that of the optimumdesigned diffractive optical element, so that it has good angularcharacteristics. Moreover, diffraction efficiency is always 0.97 or moreat d-line and g-line, has little spectral dependency, and does not haveany sharp drop with respect to wavelength, so that the diffractiveoptical element 1 also has very good spectral characteristics.

[0095] Although the above-mentioned examples relate to the multi-layertype diffractive optical element composed of two layers separated witheach other, the present invention can be applied to a multi-layer typediffractive optical element composed of two layers closely laminatedwith each other or that composed of three or more layers separated orclosely laminated with each other.

[0096]FIG. 2 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated three layers according tothe first embodiment of the present invention. A first diffractiveelement 30 whose light-exit-side surface is formed in a saw-tooth shape,a second diffractive element 40 closely laminated with the saw-toothshape surface of the first diffractive element 30, and a thirddiffractive element 50 closely laminated with the light-exit-sidesurface of the second diffractive element 40. These first, second, andthird diffractive elements 30, 40, and 50 are made of differentmaterials with each other, so that they have different refractiveindices and dispersion with each other.

[0097] In a diffractive optical element having such structure, grooveheights d1S, d2S, and d3S can be calculated from the followingsimultaneous equations (4), (5), and (6):

d1S·(n11−1)−d2S·(n21−1)+d3S·(n31−1)=λ1   (4)

d1S·(n12−1)−d2S·(n22−1)+d3S·(n32−1)=λ2   (4)

d1S·(n13−1)−d2S·(n23−1)+d3S·(n33−1)=λ3   (4)

[0098] where d1S, d2S, and d3S (d1S, d2S, and d3S are not shown in FIG.2) denote optimum designed groove heights of the first, second, andthird diffractive elements, respectively, λ1, λ2, and λ3 denotewavelengths where chromatic aberration correction is carried out,respectively, n11 denotes refractive index of the first diffractiveelement 30 at wavelength λ1, n12 denotes refractive index of the firstdiffractive element 30 at wavelength λ2, n13 denotes refractive index ofthe first diffractive element 30 at wavelength λ3, n21 denotesrefractive index of the second diffractive element 40 at wavelength λ1,n22 denotes refractive index of the second diffractive element 40 atwavelength λ2, n23 denotes refractive index of the second diffractiveelement 40 at wavelength λ3, n31 denotes refractive index of the thirddiffractive element 50 at wavelength λ1, n32 denotes refractive index ofthe third diffractive element 50 at wavelength λ2, and n33 denotesrefractive index of the third diffractive element 50 at wavelength λ3.

[0099] The optimum designed groove heights d1S, d2S, and d3S of thefirst, second, and third diffractive elements, respectively, are derivedby solving the above simultaneous equations (4), (5), and (6). Then, thegroove heights d1, d2, and d3 of the first, second, and thirddiffractive elements, respectively, composing the diffractive opticalelement according to the first embodiment of the present invention aredetermined by using similar procedure described above in the firstembodiment.

[0100] However, in the multi-layer type diffractive optical element thatthree layers are closely laminated shown in FIG. 2, the groove height Dof the diffractive optical element according to the first embodiment ofthe present invention is the summation (D=d1+d3) of the groove height d1of the first diffractive element 30 and that d3 of the third diffractiveelement 50. The groove height (optimum designed value) DS of thestandard diffractive optical element (not shown) is the summation(DS=d1S+d3S) of the groove height (optimum designed value) d1S of thefirst diffractive element 30 and that (optimum designed value) d3S ofthe third diffractive element 50.

[0101]FIG. 3 is a sectional view showing a multi-layer type diffractiveoptical element composed of closely laminated two layers according tothe first embodiment of the present invention. A first diffractiveelement 70 whose light-exit-side surface is formed in a saw-tooth shape,and a second diffractive element 80 closely laminated with the saw-toothshape surface of the first diffractive element 70. These first andsecond diffractive elements 70 and 80 are made of different materialswith each other, so that they have different refractive indices anddispersion with each other. The way to obtain the groove heights d1 andd2 of the first and second diffractive elements, respectively, is thesame as the above-described case of the multi-layer type diffractiveoptical element 1 composed of two layers. The groove height D of thediffractive optical element according to this embodiment, however, isthe groove height d1 itself of the first diffractive element 70.Moreover, the groove height (optimum designed value) DS of the standarddiffractive optical element (not shown) according to this embodiment isthe groove height (optimum designed value) d1S itself of the firstdiffractive element.

[0102] As described above, a diffractive optical element according tothe first embodiment of the present invention can improve angularcharacteristics of diffraction efficiency relative to a conventional onewithout largely deteriorating spectral characteristics when the grooveheight is lowered to satisfy the conditional expression 0.5≦D/DS≦0.9,where DS denotes the optimum designed value of a groove height of thediffractive optical element and D denotes an actual groove height thediffractive optical element. Moreover, since the groove height D of thisdiffraction optical element becomes lower than the optimum designedvalue DS, there is an advantage to be easily manufactured.

[0103] [Second Embodiment]

[0104] A second embodiment according to the present invention isexplained below with reference to accompanying drawings. FIG. 4 is asectional view showing a multi-layer type diffractive optical elementcomposed of closely laminated two layers according to a secondembodiment of the present invention. The diffractive optical element 101according to the second embodiment of the present invention is composedof a first diffractive element 110 and a second diffractive element 120,made of different materials with each other, and closely laminated witheach other. A diffraction grating 130 is formed on the boundary betweenthe first and second diffractive elements 110 and 120. In the secondembodiment of the present invention, although the diffraction grating130 has a saw-tooth shape, the present invention is not limited to thedisclosure.

[0105] In the diffractive optical element 101 according to the secondembodiment of the present invention, one of materials composing thefirst and second diffractive elements 110 and 120 is made of glass forglass molding and a material of the other diffractive element is anultraviolet-curable resin. In the second embodiment of the presentinvention, although the diffractive optical element is explained to havethe structure that the first diffractive element 110 is made of glassfor glass molding and the second diffractive element 120 is made of anultraviolet-curable resin, the order of the materials may be reversed.In the second embodiment of the present invention, it becomes possibleto apply injection molding by using the glass for glass molding, so thatproductivity increases. Moreover, in the second embodiment of thepresent invention, an ultraviolet-curable resin is used instead of athermoplastic resin. A thermoplastic resin (MS300, OPET, and the likedisclosed in Japanese Laid-Open Patent Application No. 11-271513) has adefect that it has bad contact with glass because of large contractionrate upon molding, so that they are not readily laminated with eachother. A molding machine for a thermoplastic resin is a large scale anddoes not have good productivity. On the other hand, anultraviolet-curable resin has merit that it has good contact with glassand can be molded with a small molding machine. In the second embodimentof the present invention, since a closely laminated multi-layer typediffractive optical element as shown in FIG. 4 is used, the grooveheight of the diffraction grating can be lower than a conventionalmulti-layer type diffractive optical element as shown in FIG. 9.Accordingly, angular characteristics can be improved.

[0106] In a diffractive optical element 101 according to the secondembodiment of the present invention, the following conditionalexpression (7) is satisfied:

7.0 μm≦h≦18.0 μm   (7)

[0107] where h denotes the groove height 130 of the diffractive opticalelement 101. Conditional expression (7) defines an appropriate range ofthe groove height regarding angular characteristics (the decreasing ratein diffraction efficiency relative to angular change in incident light).When the condition is satisfied, angular characteristics can be improvedin comparison with a conventional laminated multi-layer type diffractiveoptical element. In other words, by lowering the groove height h of thediffraction grating 130 by the upper limit (18.0 μm) of conditionalexpression (7) or less, light loss upon transmitting can be lower,thereby improving the angular characteristics. However, the height hcannot be lowered without restriction because there is some possibilitythat severe manufacturing accuracy cannot be satisfied, so that thegroove height h of conditional expression (7) preferably has a lowerlimit. When the lower limit of conditional expression (7) is set to 8.0μm and the upper limit is set to 16.0 μm, better result can be obtained.

[0108] In a diffractive optical element 101 according to the secondembodiment of the present invention, the vertex angle θ (see FIG. 4) ofthe diffraction grating 130 can be gentle by widening the pitch p(minimum pitch: see FIG. 4) of the diffraction grating 130 by 70 μm ormore. When the vertex angle θ of the diffraction grating 130 is made tobe gentle as described above, by using a mold (a first mold 150) asdescribed below, the shape can precisely be transferred upon forming thefirst diffractive element 110. Moreover, since an ultraviolet-curableresin dripped onto the diffraction grating 130 transferred as describedabove spreads all over the grooves of the diffraction grating 130 formed(transferred) on the first diffractive element 110, the diffractiongrating 130 having a predetermined shape can be easily formed.Accordingly, the productivity of the diffractive optical element 101according to the second embodiment of the present invention can beincreased. When the pitch (minimum pitch) p of the diffraction grating130 is made larger to 100 μm or more, the vertex angle θ of thediffraction grating 130 become gentler, so that it becomes easier toform the diffraction grating 130. Moreover, when the pitch becomeswider, angular characteristics increase. According to our simulation, apreferable result can be obtained about 70 μm or more.

[0109] Furthermore, in a diffractive optical element 101 according tothe second embodiment of the present invention, glass for glass moldingcomposing the first diffractive element 110 preferably satisfies thefollowing conditional expressions (8) and (9) and an ultraviolet-curableresin composing the second diffractive element 120 preferably satisfiesthe following conditional expressions (10) and (11):

1.55≦ndG≦1.65   (8)

55 νdG≦65   (9)

1.50≦ndR≦1.60   (10)

νdR≦40   (11)

[0110] where ndG and νdG denote refractive index at d-line and Abbenumber of the glass for glass molding, respectively, and ndR and νdRdenote refractive index at d-line and Abbe number of theultraviolet-curable resin, respectively.

[0111] Alternatively, glass for glass molding preferably satisfies thefollowing conditional expressions (12) and (13) and anultraviolet-curable resin preferably satisfies the following conditionalexpressions (14) and (15):

1.63≦ndG≦1.73   (12)

50≦νdG≦60   (13)

1.58≦ndR≦1.68   (14)

νdR≦35   (15).

[0112] Both conditional expressions (8) and (9), or (12) and (13) defineappropriate ranges of glass for glass molding among various kinds ofmolding glass which is particularly compatible with theultraviolet-curable resin. When each value becomes out of each range, itbecomes difficult to obtain the multi-layer type diffractive opticalelement according to the second embodiment having a shape that the firstdiffractive element 110 (glass for glass molding) contacts with thesecond diffractive element 120 (an ultraviolet-curable resin) at thecommon diffraction grating 130. When the lower limit and the upper limitof conditional expression (8) are set to 1.57 and 1.63, respectively,and the lower limit and the upper limit of conditional expression (9)are set to 57 and 63, respectively, more preferable result can beobtained. Similarly, when the lower limit and the upper limit ofconditional expression (12) are set to 1.65 and 1.70, respectively, andthe lower limit and the upper limit of conditional expression (13) areset to 52 and 58, respectively, further more preferable result can beobtained.

[0113] Furthermore, conditional expressions (10) and (11), or (14) and(15) define appropriate ranges for keeping various characteristics ofthe diffractive optical element 101 well. When these values come outfrom the ranges of the conditional expressions, the groove height h ofthe diffraction grating 130 becomes high to result in deterioration ofangular characteristics or decrease in spectral diffraction efficiencyeven if the diffractive optical element 101 has a shape that the firstdiffractive element 110 (glass for glass molding) contacts with thesecond diffractive element 120 (an ultraviolet-curable resin) at thecommon diffraction grating 130. When the lower limit and the upper limitof conditional expression (10) are set to 1.52 and 1.58, respectively,and the upper limit of conditional expression (11) is set to 25, betterresult can be obtained. Similarly, when the upper limit of conditionalexpression (14) is set to 1.65, and the lower limit and the upper limitof conditional expression (15) are set to 20 and 30, respectively, evenbetter result can be obtained.

[0114] Then, the manufacturing procedure of the diffractive opticalelement 101 according to the second embodiment of the present inventionis explained. At first, a first mold 150 on which a predetermined shapeof a diffraction grating is formed in advance and a second mold 160 onwhich a predetermined shape of a surface is formed in advance areprepared. Glass 110A for glass molding that is formed to a predeterminedshape (e.g., a disk shape in the second embodiment) and heated more thanglass transition temperature is prepared (see FIG. 5A). As for glass110A for glass molding, those explained below in Examples arerecommended.

[0115] The above-described glass 110A for glass molding that is heatedmore than glass transition temperature is pressed with the first mold150 and the second mold 160, gradually cooled to be hardened (see FIG.5B). The hardened glass 110A for glass molding is removed from the firstmold 150 and the second mold 160 (see FIG. 5C). Accordingly, the shapeof the diffraction grating formed on the first mold 150 is transferredto the glass 110A for glass molding to form a first diffractive element110.

[0116] Then, a proper quantity of liquid type ultraviolet-curable resin120A is dropped onto a surface of the first diffractive element 110manufactured with the above-described procedure, where the diffractiongrating 130 is formed (see FIG. 5D). As for the ultraviolet-curableresin 120A, those explained below in Examples are recommended. In theliquid type ultraviolet-curable resin 120A, a third mold 170 for forminga surface is pressed to the opposite surface of a surface where thediffraction grating 130 is formed (see FIG. 5E). Then, the liquid typeultraviolet-curable resin 120A is cured by radiating it with anultraviolet light 180 (see FIG. 5F). Accordingly, the second diffractiveelement 120 that is closely laminated with the first diffractive element110 is formed. Finally, when the third mold 170 for forming a surface isremoved, the laminated multi-layer type diffractive optical element 101according to the second embodiment of the present invention composed ofthe first diffractive element 110 (glass for glass molding) and thesecond diffractive element 120 (ultraviolet-curable resin) is completed.

[0117] When the laminated multi-layer type diffractive optical element101 according to the second embodiment of the present invention ismanufactured with the procedure described above, the mold on which thediffraction grating 130 is to be formed in advance is only one (thefirst mold 150 ), so that the manufacturing cost can be reduced.Moreover, it is not necessary to adjust both diffraction gratings 130formed on the first and second diffractive elements 110 and 120. Thesecond embodiment uses diffraction efficiency (η_(A)).

EXAMPLE 6

[0118] In this example, VC78 having ndG=1.66910, νdG=55.4 (a product ofSumita Optical Glass, Inc.) is used as glass 110A for glass molding andHV16 having ndR=1.5980, νdR=28.0 (a product of ADEL CO.,LTD) is used asan ultraviolet-curable resin 120A. The groove height h of thediffraction grating 130 is 8.1 μm. In this construction, we haveobtained high diffraction efficiency of 0.97 or more from g-line toC-line.

EXAMPLE 7

[0119] In this example, a low-glass-transition-temperature glass Ahaving ndG=1.67790, νdG=55.3 is used as glass 110A for glass molding andan ultraviolet-curable resin D having ndR=1.6350, νdR=23.0 is used as anultraviolet-curable resin 120A. The groove height h of the diffractiongrating 130 is 15.0 μm. In this construction, we have obtained highdiffraction efficiency of 0.97 or more from g-line to C-line.

EXAMPLE 8

[0120] In this example, VC79 having ndG=1.60970, νdG=57.8 (a product ofSumita Optical Glass, Inc.) is used as glass 110A for glass molding andan ultraviolet-curable resin E having ndR=1.5440, νdR=29.3 is used as anultraviolet-curable resin 120A. The groove height h of the diffractiongrating 130 is 8.8 μm. In this construction, we have obtained highdiffraction efficiency of 0.98 or more from g-line to C-line.

EXAMPLE 9

[0121] In this example, a low-glass-transition-temperature glass Chaving ndG=1.59813, νdG=61.1 is used as glass 110A for glass molding andan ultraviolet-curable resin F having ndR=1.5539, νdR=38.1 is used as anultraviolet-curable resin 120A. The groove height h of the diffractiongrating 130 is 16.4 μm. In this construction, we have obtained highdiffraction efficiency of 0.98 or more from g-line to C-line.

[0122] According to above-described examples of the second embodiment,when the groove height h of the diffraction grating 130 is set to avalue satisfying conditional expression (7), we have obtained highdiffraction efficiency of 0.97 or more from g-line to C-line andconfirmed to have good angular characteristics.

[0123] As described above, the second embodiment of the presentinvention makes it possible to provide a diffractive optical elementhaving improved angular characteristics relative to a conventional oneand improved productivity.

[0124] [Third Embodiment]

[0125] A third embodiment of the present invention is explained belowwith reference to accompanying drawings. FIG. 6 is a sectional viewshowing a multi-layer type diffractive optical element composed ofclosely laminated three layers according to a third embodiment of thepresent invention. The diffractive optical element 201 according to thethird embodiment of the present invention is composed of a firsttransparent member 210 on which a diffraction grating 240 is formed, asecond transparent member 220 which has a plane or curved surface, andadhesive 230 for cementing the diffraction grating 240 of the firsttransparent member 210 onto the plane or curved surface of the secondtransparent member 220. Here, the adhesive and the first transparentmember 210 are different in their materials and have differentrefractive indices or Abbe numbers with each other. The plane or curvedsurface of the second transparent member 220 does not have grooves of adiffraction grating. In the third embodiment of the present invention,although the diffraction grating 240 formed on the first transparentmember 210 has a saw-tooth shape as shown in FIG. 6, the presentinvention is not limited to the disclosure.

[0126] Using this structure of the diffractive optical element 201 makesit possible to manufacture the diffraction grating 240 by using only onemold, so that the conventional procedure that two diffraction gratingsare manufactured separately and aligned their relative positions becomesunnecessary. Moreover, since the adhesive 230 is used, a diffractiveoptical element can be manufactured by putting the adhesive 230 inbetween the first transparent member 210 and the second transparentmember 220 and by curing it. In addition, since the adhesive 230 isused, it is effective to prevent the diffraction grating 240 from comingoff at a surface 270.

[0127] In the diffractive optical element 201 according to the thirdembodiment of the present invention, it is preferable that the firsttransparent member 210 is made of glass for glass molding. The glass forglass molding has a lot of kinds, so that the material has broad optionsto be easy for manufacturing. Moreover, it is preferable that theadhesive 230 is made of an ultraviolet-curable resin. Theultraviolet-curable resin performs its mission of adhering the firsttransparent member 210 to the second transparent member 220 and is ableto be cured easily by radiating it with an ultraviolet light asdescribed later.

[0128] In the diffractive optical element 201, the materials of thefirst transparent member 210 and the adhesive 230 preferably satisfy thefollowing conditional expressions (16) and (17), and (18) and (19),respectively:

1.55≦ndG≦1.70   (16)

50≦νdG≦65   (17)

1.50≦ndR≦1.65   (18)

νdR≦45   (19)

[0129] where ndG and νdG denote the refractive index at d-line and Abbenumber of the first transparent member 210, respectively, and ndR andνdR denote the refractive index at d-line and Abbe number of theadhesive 230, respectively.

[0130] Conditional expressions (16), (17), (18) and (19) defineappropriate ranges of different materials as shown in FIG. 6 (here, theyare the first transparent member 210 and the adhesive 230) to be able tocontact with each other at the common diffraction grating 240. When eachvalue becomes out of each range, it becomes difficult to obtain themulti-layer type diffractive optical element according to the thirdembodiment of the present invention. In particular, conditionalexpressions (16) and (18) are for obtaining good angularcharacteristics. When each value falls below the lower limit ofconditional expression (16) or exceeds the upper limit of conditionalexpression (18), the groove height h of the diffraction grating 240 (seeFIG. 6) becomes too high to obtain a predetermined groove shape withmaintaining angular characteristics well. Conditional expressions (17)and (19) are for obtaining good diffraction efficiency over entirewavelength range. When each value becomes out of each range, it becomesdifficult to obtain good diffraction efficiency over entire wavelengthrange.

[0131] When the lower limit and the upper limit of conditionalexpression (16) are set to 1.57 and 1.68, respectively, and the lowerlimit and the upper limit of conditional expression (17) are set to 52and 63, respectively, a further preferable result can be obtained.Similarly, when the lower limit and the upper limit of conditionalexpression (18) are set to 1.52 and 1.63, respectively, and the lowerlimit and the upper limit of conditional expression (19) are set to 20and 43, respectively, a further preferable result can be obtained.

[0132] Furthermore, in the diffractive optical element 201, thefollowing conditional expression (20) is preferably satisfied:

h≦16.0 μm   (20)

[0133] where h denotes the groove height of the diffraction grating 240.

[0134] Conditional expression (20) defines an appropriate range of thegroove height relative to angular characteristics (the decreasing ratein diffraction efficiency relative to angular change in incident light).When the condition is satisfied, angular characteristics can be improvedin comparison with a conventional laminated multi-layer type diffractiveoptical element. In other words, by lowering the groove height h of thediffraction grating 240 by the upper limit (16.0 μm) of conditionalexpression (20) or less, light loss upon transmitting can be lower,thereby improving the angular characteristics. However, the height hcannot be lowered without restriction because there is some possibilitythat severe manufacturing accuracy cannot be satisfied, so that thegroove height h of conditional expression (20) preferably has a lowerlimit. When the lower limit of conditional expression (20) is set to 6.5μm and the upper limit is set to 15.0 μm, better result can be obtained.

[0135] In a diffractive optical element 201 according to the thirdembodiment of the present invention, the vertex angle θ (see FIG. 6) ofthe diffraction grating 240 can be gentle by widening the pitch p(minimum pitch: see FIG. 6) of the diffraction grating 240 by 80 μm ormore. When the vertex angle θ of the diffraction grating 240 is made tobe gentle as described above, by using a mold (a first mold 250) asdescribed below, the shape can precisely be transferred upon forming thefirst diffractive element 210. Moreover, since an ultraviolet-curableresin dripped onto the diffraction grating 240 transferred as describedabove spreads all over the grooves of the diffraction grating 240 formed(transferred) on the first diffractive element 210, the diffractiongrating 240 having a predetermined shape can be easily formed.Accordingly, the productivity of the diffractive optical element 201according to the third embodiment of the present invention can beincreased. When the pitch (minimum pitch) p of the diffraction grating240 is made larger to 100 μm or more, the vertex angle θ of thediffraction grating 240 becomes gentler, so that it becomes easier toform the diffraction grating 240.

[0136] The diffractive optical element 201 according to the thirdembodiment of the present invention can be manufactured easily becausethe first transparent member 210 on which the diffraction grating 240 isformed is simply cemented to the second transparent member 220 having aplane or curved surface between the diffraction grating 240 of the firsttransparent member 210 and the plane or curved surface of the secondtransparent member 220 with the adhesive 230.

[0137] Then, the manufacturing procedure of the diffractive opticalelement 201 according to the third embodiment of the present inventionis explained. At first, a first mold 250 on which a predetermined shapeof a diffraction grating is formed in advance and a second mold 260 onwhich a predetermined shape of a surface is formed in advance areprepared. Glass 210A for glass molding that is formed to a predeterminedshape (e.g., a disk shape in the third embodiment) and heated more thanglass transition temperature is prepared (see FIG. 7A). As for glass210A for glass molding, those explained below in Examples arerecommended.

[0138] The above-described glass 210A for glass molding that is heatedmore than glass transition temperature is pressed with the first mold250 and the second mold 260, gradually cooled to be hardened (a firststep, see FIG. 7B). The hardened glass 210A for glass molding is removedfrom the first mold 250 and the second mold 260 (see FIG. 7C).Accordingly, the shape of the diffraction grating 240 formed on thefirst mold 250 is transferred to the glass 210A for glass molding toform a first diffractive element 210.

[0139] At the same time, the second transparent member 220 ismanufactured. The second transparent member 220 has a plane or curvedsurface 221where the second transparent member 220 contacts with theadhesive 230 as shown in FIG. 7E. When the material of the secondtransparent member 220 is glass for glass molding, the glass is heatedand pressed by using suitable molds to form the shape. When the materialof the second transparent member 220 is common glass, both surfaces areformed by polishing in accordance with the conventional lensmanufacturing method. When the material of the second transparent member220 is resin, both surfaces are formed by molding.

[0140] Then, a proper quantity of liquid type adhesive 230 is droppedonto the diffraction grating 240 of the first transparent member 210manufactured with the above-described procedure (a second step, see FIG.7D). As for the adhesive 230, an ultraviolet-curable resin is the mostsuitable, so those explained below in Examples are recommended. In theliquid type adhesive 230, the second transparent member 220 is pressedto the opposite surface 221 of a surface where the diffraction grating240 is formed (a third step, see FIG. 7E). Then, the liquid typeadhesive 230 is cured by radiating it with an ultraviolet light 280 (afourth step, see FIG. 7F). Accordingly, the first transparent member 210is cemented with the adhesive 230 and the second transparent member 220is cemented with the adhesive 230, so that the diffractive opticalelement 201 according to the third embodiment of the present inventionis completed (see FIG. 7G).

[0141] In the above-described manufacturing method for the diffractiveoptical element 201, although the method is for manufacturing themulti-layer type diffractive optical element, among the entire steps,the step for forming grooves of the diffraction grating is only the stepfor forming the diffraction grating on the first transparent member 210.Accordingly, it simplifies the manufacturing process in comparison withthe conventional multi-layer type diffractive optical element shown inFIG. 9 that the diffraction grating 321 must be formed separately on thesecond optical element 320 other than the diffraction grating 311 isformed on the first optical element 310. Therefore, in the manufacturingmethod for the diffractive optical element according to the thirdembodiment of the present invention, the multi-layer type diffractiveoptical element can be manufactured with low cost, which allows increasein productivity.

[0142] Moreover, when the diffractive optical element 201 according tothe third embodiment of the present invention is manufactured by theabove-described procedure, only one mold (the first mold 250) isrequired for forming the diffraction grating (here, the grooves of thediffraction grating 240), so that the closely laminated multi-layer typediffractive optical element can be manufactured with low cost.Accordingly, it is not necessary to adjust both diffraction gratingsformed on the first transparent member 210 and the adhesive 230. In thethird embodiment of the present invention, the first transparent member210 and the adhesive 230 are only required to have different refractiveindices and Abbe numbers with each other, so that it is allowed that thefirst transparent member 210 and the second transparent member 220 aremade of the same material.

[0143]FIGS. 8A and 8B are sectional views of diffractive opticalelements according to modified examples of the third embodiment of thepresent invention. FIG. 8A is a case that a surface 221 of the secondtransparent member 220 come in contact with the adhesive 230 has aconvex shape facing to the diffraction grating 240 side. FIG. 8B is acase that a surface 221 of the second transparent member 220 come incontact with the adhesive 230 has a concave shape facing to thediffraction grating 240 side.

[0144] The diffractive optical element 201 according to the thirdembodiment of the present invention makes it possible to converge aspecific order diffracted light at a point to be used as a lens. In thiscase, the diffractive optical element is made to have a disk shape as awhole. The sectional shape of the diffractive optical element accordingto the third embodiment of the present invention may be a plane parallelshape as shown in FIG. 6 or lens shapes as shown in FIGS. 8A and 8B. Thethird embodiment uses diffraction efficiency (η_(I))

EXAMPLE 10

[0145] In Example 10, VC78 having ndG=1.66910, νdG=55.4 (a product ofSumita Optical Glass, Inc.) is used as glass 210A for glass molding andHV16 having ndR=1.5980, νdR=28.0 (a product of ADEL CO.,LTD) is used asan adhesive 230 (ultraviolet-curable resin). The groove height h of thediffraction grating 240 is 8.0 μm. In this construction, we haveobtained high diffraction efficiency of 0.95 or more from g-line toC-line.

EXAMPLE 11

[0146] In Example 11, P-SK50 having ndG=1.59380, νdG=61.4 (a product ofSumita Optical Glass, Inc.) is used as glass 210A for glass molding andan ultraviolet-curable resin G having ndR=1.5499, νdR=41.6 is used as anadhesive 230. The groove height h of the diffraction grating 240 is 12.7μm. In this construction, we have obtained high diffraction efficiencyof 0.95 or more from g-line to C-line.

[0147] As described above, the third embodiment of the present inventionmakes it possible to provide a multi-layer type diffractive opticalelement capable of being manufactured easily and a manufacturing methodthereof.

[0148] Additional advantages and modification will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details, and representativedevices shown and described herein. Accordingly, various modificationsmay be made without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A diffractive optical element comprising: atleast a first diffractive element and a second diffractive element,wherein the following conditional expression is satisfied: 0.5≦D/DS≦0.9where DS denotes a summation of optimum designed groove heights of thediffractive optical elements, and D denotes a summation of actual grooveheights of the diffractive optical elements.
 2. The diffractive opticalelement according to claim 1, wherein at least one of the firstdiffractive element and the second diffractive element is made of glass.3. The diffractive optical element according to claim 2, wherein atleast one of the first diffractive element and the second diffractiveelement is made of resin.
 4. The diffractive optical element accordingto claim 3, wherein the optimum designed groove heights of thediffractive optical elements are determined so as to satisfy a conditionfor correcting chromatic aberration at both d-line and g-line.
 5. Thediffractive optical element according to claim 2, wherein the optimumdesigned groove heights of the diffractive optical elements aredetermined so as to satisfy a condition for correcting chromaticaberration at both d-line and g-line.
 6. The diffractive optical elementaccording to claim 1, wherein at least one of the first diffractiveelement and the second diffractive element is made of resin.
 7. Thediffractive optical element according to claim 6, wherein the optimumdesigned groove heights of the diffractive optical elements aredetermined so as to satisfy a condition for correcting chromaticaberration at both d-line and g-line.
 8. A diffractive optical elementcomprising: a first diffractive element and a second diffractiveelement: and the first and second diffractive elements being made ofdifferent materials with each other, closely laminated with each otherto form grooves of a diffraction grating having a predetermined shape atthe cemented surface of the first and the second diffractive elements;wherein one of these diffractive elements is made of material formolding and the other diffractive element is made of ultraviolet-curableresin; and wherein the following conditional expression is satisfied:7.0 μm≦h≦18.0 μm where h denotes the groove height of the diffractiongrating.
 9. The diffractive optical element according to claim 8,wherein the material for molding is glass and the following conditionalexpressions are satisfied: 1.55≦ndG≦1.65 55νdG≦65 1.50≦ndR≦1.60 νdR≦40where ndG denotes refractive index of the glass at d-line, νdG denotesAbbe number of the glass, ndR denotes refractive index of theultraviolet-curable resin at d-line, and νdR denotes Abbe number of theresin.
 10. The diffractive optical element according to claim 8, whereinthe material for molding is glass and the following conditionalexpressions are satisfied: 1.63≦ndG≦1.73 50νdG≦60 1.58≦ndR≦1.68 νdR≦35where ndG denotes refractive index of the glass at d-line, νdG denotesAbbe number of the glass, ndR denotes refractive index of theultraviolet-curable resin at d-line, and νdR denotes Abbe number of theresin.
 11. A diffractive optical element comprising: a first diffractiveelement and a second diffractive element: and the first and seconddiffractive elements being made of different materials with each other,closely laminated with each other to form grooves of a diffractiongrating having a predetermined shape at the cemented surface of thefirst and the second diffractive elements; wherein one of thesediffractive elements is made of material for molding and the otherdiffractive element is made of ultraviolet-curable resin; and wherein apitch of the grooves of the diffraction grating is 70 μm or more. 12.The diffractive optical element according to claim 11, wherein thematerial for molding is glass and the following conditional expressionsare satisfied: 1.55≦ndG≦1.65 55νdG≦65 1.50≦ndR≦1.60 νdR≦40 where ndGdenotes refractive index of the glass at d-line, νdG denotes Abbe numberof the glass, ndR denotes refractive index of the ultraviolet-curableresin at d-line, and νdR denotes Abbe number of the resin.
 13. Thediffractive optical element according to claim 11, wherein the materialfor molding is glass and the following conditional expressions aresatisfied: 1.63≦ndG≦1.73 50νdG≦60 1.58≦ndR≦1.68 νdR≦35 where ndG denotesrefractive index of the glass at d-line, νdG denotes Abbe number of theglass, ndR denotes refractive index of the ultraviolet-curable resin atd-line, and νdR denotes Abbe number of the resin.
 14. A diffractiveoptical element comprising: a first transparent member on which groovesof a diffraction grating is formed; a second transparent member having aplane or curved surface; and an adhesive agent for cementing the groovesof the diffraction grating of the first transparent member with theplane or curved surface of the second transparent member.
 15. Thediffractive optical element according to claim 14, wherein the firsttransparent member is made of material for molding.
 16. The diffractiveoptical element according to claim 15, wherein the adhesive agent ismade of ultraviolet-curable resin.
 17. The diffractive optical elementaccording to claim 16, wherein the following conditional expressions aresatisfied: 1.55≦ndG≦1.70 50νdG≦65 1.50≦ndR≦1.65 νdR≦45 where ndG denotesrefractive index at d-line of the first transparent member and νdGdenotes Abbe number of the first transparent member, ndR denotesrefractive index at d-line of the adhesive agent, and νdR denotes Abbenumber the adhesive agent.
 18. The diffractive optical element accordingto claim 17, wherein the following conditional expression is satisfied:h≦16.0 μm where h denotes the groove height of the diffraction grating.19. The diffractive optical element according to claim 18, wherein apitch of the grooves of the diffraction grating is 80 μm or more. 20.The diffractive optical element according to claim 16, wherein thefollowing conditional expression is satisfied: h≦16.0 μm where h denotesthe groove height of the diffraction grating.
 21. The diffractiveoptical element according to claim 20, wherein a pitch of the grooves ofthe diffraction grating is 80 μm or more.
 22. The diffractive opticalelement according to claim 15, wherein the following conditionalexpressions are satisfied: 1.55≦ndG≦1.70 50νdG≦65 1.50≦ndR≦1.65 νdR≦45where ndG denotes refractive index at d-line of the first transparentmember and νdG denotes Abbe number of the first transparent member, ndRdenotes refractive index at d-line of the adhesive agent, and νdRdenotes Abbe number the adhesive agent.
 23. The diffractive opticalelement according to claim 22, wherein the following conditionalexpression is satisfied: h≦16.0 μm where h denotes the groove height ofthe diffraction grating.
 24. The diffractive optical element accordingto claim 23, wherein a pitch of the grooves of the diffraction gratingis 80 μm or more.
 25. The diffractive optical element according to claim15, wherein the following conditional expression is satisfied: h≦16.0 μmwhere h denotes the groove height of the diffraction grating.
 26. Thediffractive optical element according to claim 14, wherein the adhesiveagent is made of ultraviolet-curable resin.
 27. The diffractive opticalelement according to claim 26, wherein the following conditionalexpressions are satisfied: 1.55≦ndG≦1.70 50νdG≦65 1.50≦ndR≦1.65 νdR≦45where ndG denotes refractive index at d-line of the first transparentmember and νdG denotes Abbe number of the first transparent member, ndRdenotes refractive index at d-line of the adhesive agent, and νdRdenotes Abbe number the adhesive agent.
 28. The diffractive opticalelement according to claim 27, wherein the following conditionalexpression is satisfied: h≦16.0 μm where h denotes the groove height ofthe diffraction grating.
 29. The diffractive optical element accordingto claim 28, wherein a pitch of the grooves of the diffraction gratingis 80 μm or more.
 30. The diffractive optical element according to claim27, wherein a pitch of the grooves of the diffraction grating is 80 μmor more.
 31. The diffractive optical element according to claim 26,wherein the following conditional expression is satisfied: h≦16.0 μmwhere h denotes the groove height of the diffraction grating.
 32. Thediffractive optical element according to claim 14, wherein the followingconditional expressions are satisfied: 1.55≦ndG≦1.70 50νdG≦651.50≦ndR≦1.65 νdR≦45 where ndG denotes refractive index at d-line of thefirst transparent member and νdG denotes Abbe number of the firsttransparent member, ndR denotes refractive index at d-line of theadhesive agent, and νdR denotes Abbe number the adhesive agent.
 33. Thediffractive optical element according to claim 32, wherein the followingconditional expression is satisfied: h≦16.0 μm where h denotes thegroove height of the diffraction grating.
 34. The diffractive opticalelement according to claim 33, wherein a pitch of the grooves of thediffraction grating is 80 μm or more.
 35. The diffractive opticalelement according to claim 32, wherein a pitch of the grooves of thediffraction grating is 80 μm or more.
 36. A method for manufacturing adiffractive optical element comprising steps of; a first step that formsgrooves of a diffraction grating on a surface of a first transparentmember; a second step that drips an adhesive agent onto the grooves ofthe diffraction grating on the surface of the first transparent member;a third step that attaches a second transparent member having a plane orcurved surface to the adhesive agent; and a fourth step that cures theadhesive agent.
 37. The method for manufacturing a diffractive opticalelement according to claim 36 wherein the first step is composed of; ahardening step that the first transparent member is pressed, formed andhardened by using a mold on which the grooves of the diffraction gratingare formed; and a removing step that removes the hardened firsttransparent member from the mold.
 38. The method for manufacturing adiffractive optical element according to claim 37 wherein the fourthstep is composed of an irradiating step that irradiates the adhesiveagent with an ultraviolet light.
 39. The method for manufacturing adiffractive optical element according to claim 36 wherein the fourthstep is composed of an irradiating step that irradiates the adhesiveagent with an ultraviolet light.